So I've accidentally given someone a BTC address and she sent BCH to this address. The wallet was created post-fork. Is it possible to go through the same process as accessing BCH immediately after the hard fork, or are these BCH funds lost forever?
If the seed phrase was generated by a BTC BIP-0039 compliant wallet and you have a BIP-0039 compliant BCH wallet then the answer is yes you should be able to get hold of your BCH funds.
It should be easy to try. Get a BCH wallet and enter your seed phrase and see if your coins appear.